The present invention relates to microelectromechanical system (MEMS) pumping devices, and more particularly to low-power, distributed MEMS pumping devices that are electrostatically actuated and the associated methods of using such devices.
Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (Micro Electro Mechanical System) structures. MEMS structures are typically capable of motion or applying force. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size.
Design freedom afforded to engineers of MEMS devices has led to the development of various techniques and structures for providing the force necessary to cause the desired motion within microstructures. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have also been successfully been used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices. One such device is found in U.S. Pat. No. 5,475,318 entitled xe2x80x9cMicroprobexe2x80x9d issued Dec. 12, 1995 in the name of inventors Marcus et al., which leverages thermal expansion to move a microdevice. A micro cantilever is constructed from materials having different thermal coefficients of expansion. When heated, the bimorph layers arch differently, causing the micro cantilever to move accordingly. A similar mechanism is used to activate a micromachined thermal switch as described in U.S. Pat. No. 5,463,233 entitled xe2x80x9cMicromachined Thermal Switchxe2x80x9d issued Oct. 31, 1995 in the name of inventor Norling.
Electrostatic forces have also been used to move structures. Traditional electrostatic devices were constructed from laminated films cut from plastic or mylar materials. A flexible electrode was attached to the film, and another electrode was affixed to a base structure. Electrically energizing the respective electrodes created an electrostatic force attracting the electrodes to each other or repelling them from each other. A representative example of these devices is found in U.S. Pat. No. 4,266,339 entitled xe2x80x9cMethod for Making Rolling Electrode for Electrostatic Devicexe2x80x9d issued May 12, 1981 in the name of inventor Kalt. These devices work well for typical motive applications, but these devices cannot be constructed in dimensions suitable for miniaturized integrated circuits, biomedical applications, or MEMS structures.
MEMS electrostatic devices are used advantageously in various applications because of their extremely small size. Electrostatic forces due to the electric field between electrical charges can generate relatively large forces given the small electrode separations inherent in MEMS devices. An example of these devices can be found in U.S. patent application Ser. No. 09/345,300 entitled xe2x80x9cARC resistant High Voltage Micromachined Electrostatic Switchxe2x80x9d filed on Jun. 30, 1999 in the name of inventor Goodwin-Johansson and U.S. patent application Ser. No. 09/320,891 entitled xe2x80x9cMicromachined Electrostatic Actuator with Air Gapxe2x80x9d filed on May 27, 1999 in the name of inventor Goodwin-Johansson. Both of these applications are assigned to MCNC, the assignee of the present invention.
It would be advantageous to develop MEMS pumping devices using electrostatic actuation that are capable of providing both large displacements of matter (typically liquid but also gasses and semi-liquid/semi-solid compositions) and large forces. The electrostatic nature of the MEMS pumping device will allow for relatively low power consumption and, therefore, no unwarranted heating of the flowing gas or fluid would occur. Additionally, the electrostatic pumping device will provide for relatively fast operation, allowing for more precise control of the pumped volume and pumping rate. In addition, it would be advantageous to develop a MEMS pumping device that allows for flow in a single predetermined direction.
Additionally, a need exists to provide for MEMS pumping devices that are capable of being used in unison to provide highly directional flow in a predetermined direction and are also capable of being patterned in an array on a substrate so as to provide for comprehensive pumping of the fluid or gas. For example, by providing for pumping devices that can be shaped and oriented on the substrate it is possible to selectively power the different pumping elements in a predetermined sequence to result in fluid or gas flow in a desired direction. This type of highly directional flow is desired in many applications, including biomedical applications and the like. Additionally, by developing a MEMS pumping device capable of being distributed in patterned arrays over the entire interior surface of a chamber or conduit it is possible to effectively pump the entire matter since the boundary of the matter is moving where the drag force exists. The individual pumping device elements of an array could be individually addressable so that the pumping matter can be directed in different directions as the application warrants.
As such, MEMS electrostatic pumping devices that have improved performance characteristics are desired for many applications. For example, MEMS pumping devices capable of fast actuation, large pumping force and large displacements that utilize minimal power are desirable, but are currently unavailable. Such devices have immediate need in those applications that desire highly directed flow, comprehensive pumping throughout an enclosed region or the ability to change flow directions by sequencing the activation of the pumping devices.
The present invention provides for improved MEMS electrostatic pumping devices that can provide large pumping force, fast actuation and large displacement of pumped matter. Further, methods for using the MEMS pumping devices according to the present invention are provided.
A MEMS pumping device driven by electrostatic forces according to the present invention comprises a substrate having at least one substrate electrode disposed thereon. Affixed to the substrate is a moveable membrane that generally overlies the at least one substrate electrode. The moveable membrane comprises at least one electrode element and a biasing element. The moveable membrane includes a fixed portion attached to the substrate and a distal portion extending from the fixed portion and being moveable with respect to the substrate electrode. A dielectric element is disposed between the at least one substrate electrode and the at least one electrode element of the moveable membrane to provide for electrical isolation. In operation, a voltage differential is established between the at least one substrate electrode and the at least one electrode element which displaces the moveable membrane relative to the substrate to thereby controllably distribute matter residing between the substrate and the distal portion of the moveable membrane.
In a further embodiment of the invention the MEMS pumping devices comprises two moveable membranes adjacently positioned on the substrate so as to impart greater desired directional pumping capability. The moveable membranes may comprise more than one electrode element. Multiple electrode elements may be individually and sequentially biased to impart greater control of directional pumping capability. The fixed portion of the moveable membranes may be limited to a corner of the membrane to allow for the pumping cavity to fill from an upstream edge of the membrane and thereby impart greater overall net flow in the desired direction.
In another embodiment of the invention the MEMS pumping device comprises one rectangular plan view shaped moveable membrane and two triangular plane view shaped moveable membranes disposed adjacent to opposite sides of the rectangular plan view shaped membrane. The moveable membranes may comprise more than one electrode element. Multiple electrode elements may be individually and sequentially biased to impart greater control of directional pumping capability. The individual moveable membranes may be sequentially biased to impart greater control of directional pumping capability.
In yet another embodiment of the invention the MEMS pumping device comprises two rectangular plan view shaped moveable membrane and two triangular plane view shaped moveable membranes disposed adjacent to opposite sides of the rectangular plan view shaped membranes. The moveable membranes may comprise more than one electrode element. Multiple electrode elements may be individually and sequentially biased to impart greater control of directional pumping capability. The fixed portion of the moveable membranes may be limited to a corner of the membrane to allow for the pumping cavity to fill from an upstream edge of the membrane and thereby impart greater overall net flow in the desired direction.
The invention is also embodied in a MEMS pumping device array that incorporates more than one MEMS pumping device disposed on a substrate. The array may be configured so that it maximizes pumping force and requisite unidirectional or multidirectional pumping direction. The substrate will typically be flexible so that it may line or form the interior walls of a conduit, chamber or the like.
In yet another embodiment, the invention comprises a method for using a MEMS pumping device. The method comprises biasing a first electrode element in a MEMS electrostatic moveable membrane. The first electrode is disposed along an upstream flow edge of the moveable membrane creating an xe2x80x9cattachedxe2x80x9d edge. The biasing of the first electrode element is followed by biasing at least one second electrode element in the MEMS electrostatic moveable membrane. The at least one second electrode element is disposed in a distal portion of the moveable membrane. Once the moveable membrane has been fully biased the release process involves releasing bias on the first electrode element while maintaining bias on the at least one second electrode element. Releasing bias on the first electrode element allows for the pumped matter (e.g. fluids or gasses) to fill the pump region from the upstream flow edge of the moveable membrane. Lastly, bias is released on the at least one second electrode to allow for the matter to fully fill the pump region.
The MEMS electrostatic pumping devices of the present invention have improved performance characteristics that are highly desirable for many micro applications. The MEMS pumping devices of the present invention are capable of fast actuation, large pumping force and large displacements while utilizing minimal power. Such devices have immediate need in those applications that desire highly directed flow, comprehensive pumping throughout an enclosed region and/or the ability to change flow directions by sequencing the activation of the pumping devices.